Use of casting compounds based on epoxy resins for encapsulating or coating electric or electronic components is known. Examples of such components include high-voltage ignition coils, electronic circuits or semiconductor elements such as diodes, silicon wafer chips or sensors. The casting compounds must comply with many different requirements in different combinations, depending on the application. In particular these casting compounds should:
have good flow properties at conventional processing temperatures, i.e., at room temperature and elevated temperatures up to approximately 80.degree. C., to permit mass production with short cycle times; PA1 be able to withstand high thermal stresses, i.e., have the highest possible glass transition temperature; PA1 adhere well to the substrates even with frequent temperature changes, in which regard a low coefficient of thermal expansion and a high internal flexibility can make a contribution; PA1 have good dielectric properties, in particular low dielectric losses, and a high dielectric strength, at least when used in the high-voltage range, for which purpose they should be free of ionic impurities as much as possible and should have low alkali content in particular; PA1 be resistant to various environmental influences at ambient temperature and optionally at high operating temperatures, e.g., resistant to fuels, oils and salt spray as well as oxidative degradation due to atmospheric oxygen; and PA1 provide effective corrosion protection for metallic substrates.
U.S. Pat. No. 3,849,187 describes encapsulating agents for transistors and other semiconductor elements that contain an epoxy resin and an amine, a phenolic novolak resin or a carboxylic anhydride hardener plus 0.1% to 5% of an added alkylalkoxysilane having two or three low alkoxy groups and one amino or epoxy group in the alkyl moiety. The silane should improve the insulation properties and endurance of the encapsulating agent and should make pretreatment or passivation of the transistor or semiconductor element superfluous. One disadvantage of these encapsulating agents is their poor thermal cycling behavior due to their greater brittleness.
German Patent No. DE 32 29 558 describes impregnating casting compounds for electric components consisting of an epoxy resin based on bisphenol A and a cycloaliphatic epoxy, a modified dicarboxylic anhydride hardener, an imidazole accelerator and a chalk filler. These impregnating compounds are not fully satisfactory because of their relatively low glass transition temperature, relatively high coefficient of thermal expansion and relatively low flexibility. Therefore, the cured impregnating compounds tend to shrink, crack and adhere poorly.
German Patent No. DE 39 13 488 describes casting compounds for electric and electronic components containing a cycloaliphatic epoxy resin having a low viscosity and the following formula: ##STR1## or a certain polyfunctional epoxy novolak resin; methylene-endo-methylenetetrahydrophthalic anhydride (also called methylnadic anhydride) as the hardener; a curing accelerator based on imidazole; amorphous silicon dioxide, optionally pretreated with an epoxysilane and having a certain particle size; a coloring agent and finally hollow glass beads as a filler. When used for components exposed to high stresses, the long-term thermal properties of the impregnating compounds and the thermal cycling behavior are inadequate.
These known casting compounds based on epoxy resins are free of silicone compounds. U.S. Pat. No. 4,560,716 discloses epoxy resin blends containing (a) an epoxy resin, (b) a hardener, (c) an additive to prevent rust such as dithiophosphoric acid, certain dithiophosphoric acid derivatives and/or certain metal salts of dithiocarbamic acids or oxidized waxes. These epoxy resin blends (d) may also contain a second additive having an alkylarylsilsesquioxane and/or an organosiloxane polymer. The following general formula is given for the alkylarylsilsesquioxane: ##STR2## where R.sup.1 through R.sup.6 may be the same or different and denote alkyl, aryl, alkenyl or aralkyl groups. The organosiloxane polymers should conform to the general formula: ##STR3## where R.sup.1 through R.sup.16 stand for alkyl, aryl, alkenyl, alkaryl groups, hydrogen or a hydroxyl group, and n and m stand for 0 or an integer of 1 or higher. The above-mentioned publication thus describes epoxy resin blends containing cyclic or open-chain silicone resins but not containing elastomeric polyorganisiloxane compounds. These epoxy resin blends are recommended for use as encapsulating materials for electric components. Their rust-preventing effect is based on the fact that after encapsulation, additives which do not react with the epoxy resin migrate to the surface of the encapsulated component, where they form a rust-preventing film. The organosilicon materials named as optional additives also prevent the loss of electric insulation properties.
German Patent No. DE 36 34 084 discloses a modified reaction resin blend that can be processed to thermoset plastics resins and may contain an epoxy resin as the reaction resin and also three-dimensionally cross-linked polyorganosiloxane rubbers in the form of particles with a diameter of 0.01 to 50 .mu.m having reactive groups at their surface which react chemically with the reaction resin, optionally in the presence of additives which serve as reaction mediators. According to this publication, these reaction resin blends are suitable in particular for production of optionally molded, high-impact thermoset plastics with good fracture toughness, fiber-reinforced plastics, insulation materials in electrical engineering and molded laminates. However, they are not recommended as casting compounds for electronic or electrotechnical components.